Change to accesskey handling

[Simon Bünzli]

Bug 340902 [1] adds two new general preferences which allow to specify
accesskey modifiers in a more accurate way:

* ui.key.contentAccess
* ui.key.chromeAccess

The .contentAccess pref contains the modifier for all content related
accesskey handling (mostly HTML accesskeys) whereas the .chromeAccess
pref is set to the modifier for all chrome related accesskey handling
(mostly XUL accesskeys).

The value of both prefs is the number of one or several of the following
modifiers:

1 = Shift
2 = Ctrl
4 = Alt
8 = Meta

These values can be combined (e.g. 5 = Alt+Shift). Setting a pref to 0
disables all respective accesskey.

For backward compatibility reasons, both prefs can be overwritten by

* ui.key.generalAccessKey

which now defaults to -1 (= use the new prefs). Gecko users who prefer
to use this pref will have to override that value with the former
default (12 for Shift, 17 for Ctrl, 18 for Alt, 224 for Meta).

The default values for the new prefs are:

Mac/Unix:
ui.key.contentAccess = 3 (Ctrl+Shift)
ui.key.chromeAccess = 2 (Ctrl)

Windows:
ui.key.contentAccess = 5 (Alt+Shift)
ui.key.chromeAccess = 4 (Alt)

I.e. no change for chrome accesskeys, but the requirement for the
additional Shift key for content accesskeys.

Feedback to this change please either in this thread or in bug 340902.

Cheers,
Simon

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=340902